Of radioactive transition metals used in radiopharmaceuticals, Tc-99m is a nuclide most often used in the field of radiodiagnostic imaging because it is advantageous, for example, in that since the energy of γ-rays emitted by Tc-99m is 141 keV and the half-life of Tc-99m is 6 hours, Tc-99m is suitable for imaging, and that Tc-99m can easily be obtained by means of a 99Mo—99mTc generator. It is considered that if a physiologically active substance or the like can be attached to this nuclide without impairing the activity, the resulting compound is useful as a diagnostic agent or a therapeutic agent.
The attempts described below were made to achieve such attachment. Transition metal nitride complexes are excellent in stability to hydrolysis. Therefore, when a transition metal nitride complex is subjected to exchange reaction with any of various ligands having a useful physiological activity, when used in a pharmaceutical, the nitride group of the nitride complex can remain bonded strongly to the metal atom. Accordingly, technetium nitride complexes having various substituents have been proposed. For example, WO 90/06137 discloses diethyl bisdithiocarbamate-Tc nitride complex, dimethyl bisdithiocarbamate-Tc nitride complex, di-n-propyl bisdithiocarbamate-Tc nitride complex, N-ethyl-N-(2-ethoxyethyl) bisdithiocarbamate-Tc nitride complex, etc. In addition, WO 89/08657, WO 92/00982, WO 93/01839 and the like disclose processes for producing a technetium nitride complex which comprises reacting a polyphosphine or the like as a reducing agent for technetium with technetium oxide, then reacting a nitride of a metal or ammonium as a nitrogen source for nitride with the reaction product to convert it to the corresponding nitride, and then coordinating a physiologically active monoclonal antibody or the like with this nitride.
In these processes, the choice of the physiologically active ligand is so important that it determines properties of the resulting pharmaceutical. But, the metal nitride complex can have various numbers of coordination positions from monodentate to tetradentate and hence is formed in plural forms. Therefore, it has been difficult to obtain a single complex stoichiometrically having a specific physiologically active ligand.
WO 98/27100 discloses that when a disphosphine compound is coordinated at two of the four coordination positions of technetium-99m nitride and a bidentate ligand having an electron-donating atom pair is coordinated at the remaining two coordination positions, the bidentate ligand is stoichiometrically coordinated, so that a single technetium-99m nitride heterocomplex can be stably obtained. However, no technetium-99m nitride heterocomplex formed by coordination of a specific bidentate ligand having a useful physiological activity has yet been obtained. Furthermore, no technetium-99m nitride heterocomplex has yet been obtained which is accumulated in specific organs, in particular, heart and adrenal glands and is accumulated in these organs in a higher proportion than in other organs, resulting in a clear distinction between an image obtained and a background.